Method and apparatus for liquid development of electrostatic images by immersion



p 23, 9 .J. HANSON 3,468,693

METHOD AND APPARATUS LIQUID DEVELOPMENT OF ELECTROSTATIC IMAGES BY MERSION Filed Apr 4. 1966 A? /Z I v 4 A mvsmon I WALTER J- HANSON United States Patent METHOD AND APPARATUS FOR LIQUID DE- VELOPMENT 0F ELECTROSTATIC IMAGES BY IMMERSION Walter J. Hanson, Old Greenwich, Conn., assignor to Pitney-Bowes, Inc., Stamford, Conn., a corporation of Delaware Filed Apr. 4, 1966, Ser. No. 539,783 Int. Cl. Bb 5/00; C23d 5 08 U.S. Cl. 117-37 9 Claims ABSTRACT OF THE DISCLOSURE Method and apparatus for extending the developing path of liquid toners in an electrostatic copy machine. The electrostatic images on a carrier are developed by immersing the charge bearing carrier into a bath of liquid developer. As the sheet is removed from the bath and while it travels to rollers which remove the excess liquid a solid plate is spaced adjacent the liquid bearing sheet to maintain a uniform film of liquid on the sheet. The spaced plate may be incorporated in the cover of the apparatus.

This invention relates to an improved developing method and apparatus. It more particularly relates to an improvement in the application of liquid developer and in the toner tank structure of electrophotographic copying machines using photoconductive materials.

Copying machines known generally as electrostatic are well known for making copies of original documents. In such machines, a uniform electrostatic charge is placed on a photoconductive coating on a carrier on which the copy is to be reproduced. The electrostatic charge may be put on the coating by any suitable manner well known in the art such as by means of a corona discharge unit. After the coating has been charged, it is exposed to a light pattern corresponding to the image of the original document to be produced. When the charged coating is exposed to the light pattern, the electrostatic charge flows from the areas struck by light leaving a latent electrostatic image in the areas which are not struck by light and still retain the charge.

In order to develop the latent electrostatic image, suitable charged toner particles are applied to the surface which adhere in the image area to form a copy image. The particles adhere naturally, but they may also be fused later by application of heat.

In the liquid toner development method, the carrier, which is usually a paper sheet is transported by rollers through a toner tank in such a manner that the sheet passes beneath the surface of the toner liquid as the paper moves through the tank. The development time, that is the time available for applying toner particles to the latent electrostatic image, is limited to the time the carrier sheet is immersed in the developing liquid. This is further dependent on space limitations within the machine, since the paper must be developed in the tank and dried afterward before leaving the machine.

Toner liquid developers are generally composed of a small amount of finely divided particles suspended in a clear dielectric liquid. The particles may be carbon black, colored resins or dye pigments. The particles must have a charge opposite in polarity to the charge placed on the copy paper. In Electrofax copying, a negative electrostatic charge is generally placed on the copy sheet having a photoconductive coating, e.g. zinc oxide, so that the toner particles will be required to have a positive charge. The liquid carrier is a suitable dielectric liquid such as for example cyclohexane, kerosene, xylene or other suitable hydrocarbon liquids. It will be understood that any suitable liquid toners can be used with the invention. The toner particles are applied as a suspension in the liquid.

The carrier paper, after leaving the developing tank passes between a pair of rollers which continue to transport the paper away from the developing tank. In addition, this pair of rollers also applies a squeegee action to the paper to remove liquid and aid in drying. The liquid squeezed by the rollers runs down the surface of the carrier over the developed image, back to the tank. This liquid flowing back can form ripples and result in an undesirable swirl pattern on the developed copy.

It is therefore an object of my invention to provide a method and apparatus for extending the developing path in an electrostatic copy machine with liquid toner development.

Another object of my invention is to provide a method and means in an electrostatic copy machine for eliminating swirl patterns on the developed image.

A further object is ot provide an improved toner tank in an electrostatic copy machine.

These and other objects of my invention will become apparent as the description thereof proceeds.

The above objects are attained by the use of the present invention. Briefly the invention consists of providing a plastic plate or other suitable material extending from just below the surface of the liquid to the vicinity of the squeegee rollers and lying closely above the path of the copy paper being developed. The spacing is such that a layer of developer liquid is maintained between the plastic plate and the copy paper, the layer including liquid carried up from the tank by the moving carrier, and liquid running back from the squeegee action of the rollers. Thus the liquid paper contact is extended. Moreover, since the liquid is maintained at a constant distance by the plate, there is no opportunity for ripples to develop and thus no swirl patterns form.

My invention may be better understood by reference to the drawings in which:

FIGURE 1 is a schematic cross-sectional view in elevation illustrating a toner tank embodying my invention.

FIGURE 2 is a perspective view of the tank in FIG- URE 1.

FIGURE 3 is a schematic cross-sectional view of an alternate embodiment of the inventive toner tank.

FIGURE 4 is a perspective view of the toner tank of FIGURE 3.

Referring to FIGS. 1 and 2, a tank 1 of generally rectangular shape contains a liquid toner developer up to level L. Tank 1 is constructed of a suitable material as well known in the art, such as a clear plastic material. A lower guide rack 2 consists of ribs 3 (see FIG. 2) attached to supports 4 and 5 which rest on the top edges of tank 1. Upper rack 6 is positioned above rack 2 and consists of ribs 7, attached to transverse rod 7A and a continuous end plate 8 at the discharge end. Both racks 2 and 6 are made of suitable plastic material.

Racks 2 and 6 dip into the developer liquid below level L and form a path 9 having an entry at 10 and an exit at 11. A pair of feed rollers 1212 are positioned above the entry 10 to convey a copy sheet 13 through path 9 and thus through the developer liquid. A pair of rollers 1414 are positioned above the exit of path 9 to convey sheet 13 and further to squeeze any liquid from the sheet 13. Plate 8 extends from slightly below liquid level L to just short of rollers 1414.

In operation, a photoconductive sheet 13 bearing a latent electrostatic image on the upper surface formed in known manner by means not shown, is conveyed by rollers 1212 to the developing tank 1 and the liquid developer. Sheet 13 passes into entry 10 and beneath liquid level L guided in path 9 between racks 2 and 6. While in the developer liquid, the latent electrostatic charge image attracts toner particles from the developer liquid to the sheet surface, thus developing a visible image on the photoconductive surface of sheet 13. As sheet 13 advances through path 9, it passes upward out of exit 11 and into the nip of rollers 1414. Rollers 14-14 squeeze excess developer liquid from sheet 13 which runs down the upper surface of sheet 13 towards the liquid level L which could customarily cause ripples in the image. Plate 8 lies closely adjacent to the moving sheet 13. Thus sheet 13 draws liquid up above the liquid level L by a meniscus action forming a continuous fluid zone F between plate 8 and sheet 13 extending from liquid level L up to rollers 14-14. Fluid zone F is also fed by liquid being squeezed by rollers 14-14 and flowing back down the surface of sheet 13. Because place 8 is in contact with fluid zone F, the fluid will always be of constant depth.

Thus fluid zone F serves to extend the effective length of developer path 9 above level L. In addition, the backwash of depleted toner liquid serves to aid in removing toner particles which may have deposited on the nonimage areas of the paper thus eliminating background and providing a print with improved contrast.

FIGS. 3 and 4 show another embodiment of the invention wherein the toner tank is enclosed at the top except at the point for entry and exit of the paper.

The tank of FIG. 3 is basically similar to that of FIG. 1 in structure and function. A cover 20 extends around the outer edges of tank 1 and below the paper path and supports the lower arcuate guide rack 2 which consists of separate ribs 3. A cover 21 extends across the top of tank 1 above the paper path and is supported by cover 20 to which it is attached by suitable means such as screws, not shown. The upper guide rack 22 consists of plates 23 dependent from the horizontal cover 24. At the entry to the tank 1, cover has a transverse ridge 25 and cover 21 has a series of curved tabs 26 which aid in guiding paper 13 from rollers 1212 into the entry 10 of path 9. At the exit end 11 of path 9, cover 24 has a transversely depressed portion 27 which dips downwardly to below the surface of liquid level L for a sufiicient distance to maintain continuous contact with level L for any minor variations in level L, then continues upwardly to form the continuous end plate 8. Thus toner tank 1 is covered except for entry 10 and exit 11 and the evaporation of toner liquid is kept to a minimum.

The operation and use of the toner tank of FIG. 3 is the same as that of FIG. 1. An agitator or stirrer is generally provided inside tank 1 to maintain the pigment particles in suspension in the toner liquid, but this has not been illustrated for purposes of clarity. Moreover, it will be understood that level L may be maintained by any known means in the art.

While certain specific embodiments of the invention have been disclosed for the sake of illustration, it will be understood that the invention is not limited thereby and that various changes and modifications of the invention may be made without departing from the spirit of the disclosure or the scope of the following claims.

What is claimed is:

1. A method for developing a latent electrostatic image on a carrier, which comprises,

passing a carrier bearing an electrostatic image on its surface through a body of liquid toner developer to develop the latent image;

forming a developer film of constant thickness on the carrier surface in contact with the image as the carrier emerges upwardly from the liquid developer body;

at a point spaced above the surface of the liquid developer body, removing liquid developer from the carrier surface; and

flowing the removed liquid developer downwardly over the carrier surface to assist in the formation of the developer film uniformly from the surface of the liquid developer body substantially to the point of liquid developer removal.

2. In a method for developing a latent electrostatic image on a carrier by contact with a liquid developer and thereafter removing the developer, the steps comprising,

passing the carrier having a latent electrostatic image thereon through a body of liquid toner developer to develop said latent image;

passing said latent image in close adjacency to a substantially flat plate to produce a meniscus of developer between said carrier and plate to form a developer film of constant thickness on the surface of said carrier in contact with the developed image as said carrier emerges from said liquid developer body; and

maintaining said image adjacent said plate to maintain said film constant on saidcarrier substantially until it reaches the point of developer removal.

3. The method of claim 2 wherein said latent electrostatic image is on the upper side of said carrier as it passes through said toner body.

4. In an apparatus for developing a latent electrostatic image on a carrier with a liquid toner developer, comprising means for containing a body of liquid toner;

means for guiding said carrier through said toner; and

means for removing excess toner from the carrier surface having the electrostatic image thereon;

the improvement comprising,

means extending between the surface of said toner and the toner removing means to maintain a uniform layer of toner liquid on said carrier surface after development of said image substantially until removal of excess toner.

5. In an apparatus for the development of a latent electrostatic image on the surface of a carrier with a liquid toner developer, comprising,

a tank for containing a body of said toner and having entry and exit points for the carrier;

guide means in said tank for guiding a carrier through said body of toner; and

guide rollers at said entry and exit points of said tanks engaging said carrier;

the improvement which comprises;

a solid plate extending from the surface of said body of toner substantially to said rollers at said tank exit point and positioned to be closely adjacent to the carrier surface having the electrostatic image thereon as the carrier emerges from said toner, whereby a uniform film of toner is maintained on the carrier from the surface of said toner body substantially to said rollers at said tank exit point.

6. The apparatus of claim 5 wherein said guide means includes upper and lower arcuate members spaced apart, and said plate is an extension of said upper guide members.

7. The apparatus of claim 5 wherein said toner tank includes a cover with entrance and exit openings, and said plate is a part of said cover adjacent said exit means.

8. An apparatus for the development of a latent electrostatic image on a carrier surface with a liquid toner developer, comprising a tank for containing a body of toner;

said tank having a first cover means around the periphery thereof;

a second cover means fitted within said first cover means, entry and exit openings between said second cover means and adjacent areas of said first cover means, a lower arcuate guide means in said tank attached to said first cover means, an upper arcuate guide means attached to the lower side of said second cover means;

said entry and exit openings and upper and lower guide means forming a path through said tank for guiding the carrier through the body of toner;

guide rollers at said entry and exit openings of said tank for engaging the carrier; said second cover means being extended downward to the surface of said body of toner at said exit opening; and

upward along said carrier path substantially to said rollers at said exit opening and positioned to be closely adjacent to the carrier passing through the toner body, whereby a uniform film of toner is maintained on the carrier surface having the electrostatic image thereon from the surface of the toner body substantially to said rollers at said exit opening.

9. The apparatus of claim 8 wherein the guide rollers at said exit opening comprise a pair of opposed rollers positioned to form a nip to squeeze excess toner liquid from the carrier.

References Cited UNITED STATES PATENTS 478,282 7/1892 Heathfield 118-419 FOREIGN PATENTS 2/1962 France. 12/1962 Great Britian.

WILLIAM D. MARTIN, Primary Examiner E. J. CABIC, Assistant Examiner US. Cl. X.R. 

